The Snohomish County Medical Examiner said a condition called “hypertrophic cardiomyopathy” caused his death. It is described by medical experts as a condition in which the heart muscle becomes thick without any obvious signs. The thickening prevents it from pumping effectively and can cause sudden cardiac arrest.

Meadowdale staff called 911 just before 10 a.m. Friday, according to school and fire officials.

Teachers began CPR, and firefighters took over. The boy was unresponsive when crews arrived. He was taken to Swedish/Edmonds hospital.